Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high performance silver halide photography. Kofron et al disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having a diameter of at least 0.6 .mu.M and a thickness of less than 0.3 .mu.m exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. In the numerous emulsions demonstrated one or more of these numerical parameters often far exceeded the stated requirements. Kofron et al recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). Spectral sensitizations in all portions of the visible spectrum and at longer wavelengths were addressed as well as orthochromatic and panchromatic spectral sensitizations for black-and-white imaging applications. Kofron et al employed combinations of one or more spectral sensitizing dyes along with middle chalcogen (e.g., sulfur) and/or noble metal (e.g., gold) chemical sensitizations, although still other, conventional sensitizations, such as reduction sensitization were also disclosed.
An early, cross-referenced variation on the teachings of Kofron et al was provided by Maskasky U.S. Pat. No. 4,435,501, hereinafter referred to as Maskasky I. Maskasky I recognized that a site director, such as iodide ion, an aminoazaindene, or a selected spectral sensitizing dye, adsorbed to the surfaces of host tabular grains was capable of directing silver salt epitaxy to selected sites, typically the edges and/or corners, of the host grains. Depending upon the composition and site of the silver salt epitaxy, significant increases in speed were observed. The most highly controlled site depositions (e.g., corner specific epitaxy siting) and the highest reported photographic speeds reported by Maskasky I were obtained by epitaxially depositing silver chloride onto silver iodobromide tabular grains. Although Maskasky I disclosed the epitaxial deposition of silver iodobromide on silver bromide tabular grains (col. 24, lines 37 and 38, Maskasky I taught a preference for epitaxially depositing a silver salt having a higher solubility than the host tabular grains, stating that this reduces any tendency toward dissolution of the tabular grains while silver salt is being deposited (col. 24, lines 10 to 14). Maskasky I recognized that even when chloride is the sole halide run into a tabular grain emulsion during epitaxial deposition, a minor portion of the halide contained in the host tabular grains can migrate to the silver chloride epitaxy. Maskasky I offers as an example the inclusion of minor amounts of bromide ion when silver and chloride ions are being run into a tabular grain emulsion during epitaxial deposition. From the iodide levels contained in the tabular grain emulsions of Maskasky I and the investigations of this invention, reported in the Examples below, it is apparent that the epitaxial depositions of Maskasky I contained only a fraction of a mole percent iodide transferred from the host tabular grains.
Maskasky U.S. Pat. No. 4,471,050, hereinafter referred to as Maskasky II, discloses that nonisomorphic silver salts can be selectively deposited on the edges of silver halide host grains without relying on a supplemental site director. The nonisomorphic silver salts include silver thiocyanate, .beta. phase silver iodide (which exhibits a hexagonal wurtzite type crystal structure), .gamma. phase silver iodide (which exhibits a zinc blende type crystal structure), silver phosphates (including meta- and pyro-phosphates) and silver carbonate. None of these nonisomorphic silver salts exhibit a face centered cubic crystal structure of the type found in photographic silver halides--i.e., an isomorphic face centered cubic crystal structure of the rock salt type. In fact, speed enhancements produced by nonisomorphic silver salt epitaxy have been much smaller than those obtained by comparable isomorphic silver salt epitaxial sensitizations.